As shown in the figure, the bullet with mass m penetrates the wood block on the smooth horizontal ground horizontally at the speed of V 0. The length of the wood block is l, the mass is m, and the resistance of the wood block to the bullet is constant. After the bullet passes through the wood block, the kinetic energy of the wood block is EK. If only the mass of the wood block or bullet changes, but the bullet can still pass through the wood block, then () A. If M is constant and M becomes smaller, the kinetic energy obtained by the wood block will certainly increase. If B. m is constant and M becomes smaller, the kinetic energy obtained by the wood block may increase. If C. m is constant and M becomes smaller, the kinetic energy obtained by the wood block will certainly increase. If D. m is constant and M becomes smaller, the kinetic energy obtained by the wood block may increase

As shown in the figure, the bullet with mass m penetrates the wood block on the smooth horizontal ground horizontally at the speed of V 0. The length of the wood block is l, the mass is m, and the resistance of the wood block to the bullet is constant. After the bullet passes through the wood block, the kinetic energy of the wood block is EK. If only the mass of the wood block or bullet changes, but the bullet can still pass through the wood block, then () A. If M is constant and M becomes smaller, the kinetic energy obtained by the wood block will certainly increase. If B. m is constant and M becomes smaller, the kinetic energy obtained by the wood block may increase. If C. m is constant and M becomes smaller, the kinetic energy obtained by the wood block will certainly increase. If D. m is constant and M becomes smaller, the kinetic energy obtained by the wood block may increase

In the same coordinate system, draw the v-image of the bullet and the block. A, B, when the mass m of the bullet changes, because the resistance of the bullet is constant, the acceleration of the bullet will increase with the decrease of the mass, while the acceleration of the block is constant. The velocity images of the two are shown in the figure